Prepregs including UV curable resins useful for manufacturing semi-flexible PCBs

ABSTRACT

Prepregs having a UV curable resin layer located adjacent to a thermally curable resin layer wherein the UV curable resin layer includes at least one UV cured resin portion and at least one UV uncured resin as well as methods for preparing flexible printed circuit boards using the prepregs.

This is a divisional of U.S. patent application Ser. No. 14/838,098filed on Aug. 27, 2015 which is a continuation of PCT/US2015/38453 filedon Jun. 30, 2015, which claims priority to U.S. provisional applicationNo. 62/019,598, filed on Jul. 1, 2014, the specifications of each ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION (1) Field of the Invention

This invention concerns films and prepregs including UV curable resinsthat are useful in the manufacture of flexible PCBs as well as tomethods for using the films and prepregs in the manufacture of printedcircuit boards.

(2) Description of the Art

Currently, standard flex-rigid multilayer manufacturing processes useflexible polyimide sheets to overlay the copper structures and noflowprepregs. Such standard flex-rigid laminates use expensive materials.Moreover, the manufacturing method is very complex and requires multiplemanual processing steps.

A prior art method for constructing build-ups for an inner-layerflexible multilayer is shown in FIG. 1. The build-ups are formedstepwise by:

-   -   structuring a polyamide foil layer including a polyamide planar        core (10) a copper layers (11) on each planar polyamide sheet        surface and coverlayers (18) overlying the copper layers (11);    -   milling a c-staged prepreg layer (12) to form a space (13);    -   applying a cover layer (14);    -   laminating the partial layup;    -   depth milling at the core top and bottoms (shown by black        triangles (16));    -   stacking-up and lamination of the multilayer; and    -   removing the rigid portion of the cores overlying space (13) by        milling.

The prior art laminate includes a plated through hole (PTH) (30). Thearea of the plated through hole in the vicinity of the structuredpolyamide foil layer creates reliability problems because it requiresplasma cleaning. In addition, issues with inadequate metallization ofthe PTH are common and undesirable barrel cracks can form due to thehigh thermal expansion of the polyimide and the cover layer incomparison the PTH thermal expansion. Moreover, during the laminationprocess, little to no pressure is applied to cover layer (14) andpolyimide foil layer (10). As a result, PTFE inlays are placed in thestack by hand to increase the stack laminating pressure.

Instead of using the polyamide foil layer (10), a cheaper FR4 materiallayer can be used instead such as is shown in FIG. 2. However thedisadvantage of using the FR4 material is that the resulting layup canonly be bent a few times before it becomes damaged. There is a need,therefore, for improved methods for manufacturing flexible multilayerinner-layer laminates that are used to manufacture printed circuitboards that do not suffer from one or more of the problems mentionedabove.

SUMMARY OF THE INVENTION

One or more of the shortcomings with flexible laminates made withpolyimide foil layers that are noted above can be overcome by replacingthe polyimide foil layer with a prepreg comprising a partially(b-staged) or fully (c-staged) cured thermally curable resin layer andan adjacent UV curable resin layer wherein the UV curable resin layerhas at least one UV light cured resin portion and at least on UV lightuncured resin portion.

Another aspect of this invention is a laminate comprising a flexiblecore having a first planar surface and a second planar surface theflexible core including an optional copper foil layer on one or both ofthe first and second planar surfaces, a UV curable resin layer having atfirst planar surface associated with one of the planar surfaces of theflexible core wherein the UV curable resin layer has at least one UVlight cured resin portion and at least on UV light uncured resin portionand a thermally cured resin layer adhered to a second planar surface ofthe UV curable resin layer.

Yet another aspect of this invention are methods for manufacturing aflexible inner-layer that include the steps of:

-   -   (a) applying a mask to a first prepreg including a UV curable        resin layer and a thermally curable resin layer wherein the mask        is applied to the UV curable resin layer, the mask including at        least one UV light transparent portion and at least one UV light        opaque portion;    -   (b) directing UV light at the mask for a period of time        sufficient to cure the UV curable resin layer underlying the UV        light transparent portion of the mask to form a UV light treated        first prepreg having at least one cured UV resin portion and at        least one uncured UV resin portion;    -   (c) contacting a core layer with the UV light treated first        prepreg such that the at least one cured UV resin portion and        the at least one uncured UV resin portion contact the core layer        to form a layup; and    -   (d) applying heat and/or pressure to the lay-up to form a        laminated layup including a rest material portion of the        semi-flexible core layer that overlies the at least one cured UV        resin portion where the semi-flexible core layer is not        substantially laminated to the at least one cured UV resin        portion. By “not substantially laminated” we mean the peel        strength of any bond between the at least one cured UV resin        portion and the semi-flexible core layer is at least 50% and        preferably at least 90% less than the peel strength of any bond        between the at least one uncured UV resin portion and the        semi-flexible core layer for the same laminated layup.

DESCRIPTION OF THE FIGURES

FIG. 1 is a flexible layup made by a prior art method;

FIG. 2 represents an alternative to the flexible layup of FIG. 1 inwhich the polyamide foil layer is replaced with an FR4 material layer;

FIG. 3 is a side view of a prepreg (100) of this invention including aUV curable resin layer (102) located adjacent to a thermally curableresin layer (104) that is partially cured;

FIG. 4 is cross section of the prepreg of FIG. 3 and further includes amask layer (108) having a UV light transparent portion (120) and a UVlight opaque portion (122);

FIG. 5 is a partially UV cured prepreg resulting from the UV curing steprepresented by FIG. 4. In FIG. 5, the prepreg includes a UV curableresin layer that has at least one UV polymerized (cured) portion (125)that underlies the UV light transparent portion (120) of mask layer(118) and at least one uncured UV resin portion(s) (127) that underliesthe opaque portion(s) mask layer (122);

FIG. 6 is side view of a layup undergoing pin lamination where the layupincludes the partially UV cured laminate of FIG. 5;

FIG. 7 and FIG. 8 are side views of the layup during and followingmilling and rest removal steps; and

FIG. 9 is a side view of a bendable printed circuit board layup of thisinvention.

DESCRIPTION OF CURRENT EMBODIMENTS

The present invention relates to the use of special prepregs to replacethe flexible polyimide sheets currently used in some flexible printedcircuit board manufacturing processes. Laminates and printed circuitboards made with the prepregs of this invention suffer from fewerreliability problems such as barrel cracks in PTH's. In addition, nospecial plasma cleaning/desmearing steps are necessary.

Referring now to the Figures, there is shown in FIG. 3 a prepreg (100)including a UV curable resin layer (102) located adjacent to a thermallycurable resin layer (104) that is at least partially cured (b-staged)and that may, in some applications, be fully cured or c-staged. UVcurable resin layer (102) may be of any useful thickness, i.e., thickenough to cause a sufficient amount of melted resin to adhere adjacentlayers of a layup. The UV curable resin layer will typically have athickness ranging from about 2 μm to about 50 μm and more preferablyfrom about 5 μm to about 20 μm. Moreover, the UV curable resin can beapplied to an exposed planar surface of thermally curable resin layer(104) by spraying, by applying it with rollers, or by any other knownmethods.

Any UV curable resin (or photopolymer) that is capable of becoming atleast partially liquid and flowable under normal printed circuit boardlaminating conditions may be used. Examples of useful UV curable resinsinclude, but are not limited to Ultraviolet (UV) and electron beam (EB)energy-cured materials such as urethanes acrylates, polyester acrylates,amino acrylates and epoxy acrylates. In addition, the UV curablematerials may include photoinitiators and additives that enhance theperformance of the pre and post cured materials.

The properties of a photocured material, such as flexibility, adhesion,and chemical resistance can be provided by functionalized oligomerspresent in the photocurable composite. As noted above, oligomerphotopolymers are typically epoxides, urethanes, polyethers, orpolyesters, each of which provides specific properties to the resultingmaterial. Each of these oligomers is typically functionalized by anacrylate. An example shown below is an epoxy oligomer that has beenfunctionalized by acrylic acid. Acrylated epoxies are useful as coatingson metallic substrates, and result in glossy hard coatings.

Acrylated urethane oligomers are typically abrasion resistant, tough,and flexible making ideal coatings for floors, paper, printing plates,and packaging materials. Acrylated polyethers and polyesters result invery hard solvent resistant films, however, polyethers are prone to UVdegradation and therefore are rarely used in UV curable material. Oftenformulations are composed of several types of oligomers to achieve thedesirable properties for the material.

As noted above, the prepreg (100) will include a thermally curable resinlayer (104). This layer will typically have a thickness of from about 5μm to about 100 μm and more preferably about 30-60 μm.

The thermally curable resin layers may be made from resins, resinsystems or mixtures of resins that are commonly used in the manufactureof printed circuit boards. The resin(s) will typically be a thermoset orthermoplastic resin. Non-limiting examples of useful resins includeepoxy resins, cyanurate resins, bismaleimide resins, polyimide resins,phenolic resins, furan resins, xylene formaldehyde resins, ketoneformaldehyde resins, urea resins, melamine resins, aniline resins, alkydresins, unsaturated polyester resins, diallyl phthalate resins, triallylcyanurate resins, triazine resins, polyurethane resins, silicone resinsand any combination or mixture thereof.

In one aspect of this invention, the resin is or includes an epoxyresin. Some examples of useful epoxy resins include phenol type epoxyresins such as those based on the diglycidyl ether of bisphenol A, onpolyglycidyl ethers of phenol-formaldehyde novolac orcresol-formaldehyde novolac, on the triglycidyl ether oftris(p-hydroxyphenol)methane, or on the tetraglycidyl ether oftetraphenylethane; amine types such as those based ontetraglycidyl-methylenedianiline or on the triglycidyl ether ofp-aminoglycol; cycloaliphatic types such as those based on3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. The term“epoxy resin” also stands for reaction products of compounds containingan excess of epoxy (for instance, of the aforementioned types) andaromatic dihydroxy compounds. These compounds may behalogen-substituted. One class of useful epoxy-resins are those that arederivatives of bisphenol A, particularly FR-4. FR-4 is made by anadvancing reaction of an excess of bisphenol A diglycidyl ether withtetrabromobisphenol A. Mixtures of epoxy resins with bismaleimide resin,cyanate resin and/or bismaleimide triazine resin can also be applied.

The resin compositions, in addition to a base resin will typicallyinclude initiators or catalysts, one or more optional flame retardantsand solvents. The flame retardant may be any flame retardant materialthat is known to be useful in resin compositions used to manufactureprepregs and laminates use to manufacture printed circuit boards. Theflame retardant(s) may contain halogens or they may be halogen free.Alternatively, or in addition, the resins may include halogens such asbromine in their backbone structure to impart the cured resin with flameretardant properties.

The resin compositions may also include polymerization initiators orcatalysts. Examples of some useful initiators or catalysts include, butare not limited to peroxide or azo-type polymerization initiators(catalysts). In general, the initiators/catalysts chosen may be anycompound that is known to be useful in resin synthesis or curing whetheror not it performs one of these functions.

The resin compositions will include one or more solvents which aretypically used to solubilize the appropriate resin compositioningredients and/or to control resin viscosity and/or in order tomaintain the resin ingredients in a suspended dispersion. Any solventknown by one of skill in the art to be useful in conjunction withthermosetting resin systems can be used. Particularly useful solventsinclude methylethylketone (MEK), toluene, dimethylformamide (DMF), ormixtures thereof. As noted below, the resin compositions are used tomanufacture prepregs and laminates. During the manufacturing process,the reinforcing materials are impregnated with or otherwise associatedwith the resin compositions and some or most of the solvent is removedfrom the resin compositions to form the prepregs and laminates. Thus,when resin composition or laminate weight percent amounts are listedherein, they are reported on a dry-solvent free-basis unless otherwisenoted.

The resin compositions may include a variety of other optionalingredients including fillers, tougheners, adhesion promoters, defoamingagents, leveling agents, dyes, and pigments. For example, a fluorescentdye can be added to the resin composition in a trace amount to cause alaminate prepared therefrom to fluoresce when exposed to UV light in aboard shop's optical inspection equipment. Other optional ingredientsknown by persons of skill in the art to be useful in resins that areused to manufacture printed circuit board laminates may also be includedin the resin compositions of this invention.

FIGS. 4 and 5 are side view of prepreg (100) of this invention duringand after a UV light curing process. FIG. 4 is a side view of theprepreg of FIG. 3 including a mask layer (118) having a UV lighttransparent portion (120) and a UV light opaque portion (122). In FIG.4, UV light (121) having a wavelength that is selected to cure the UVcurable resin is directed at prepreg (100). The UV light passes throughtransparent portion (120) of mask (118) and cures the UV curable resinthat underlies transparent portion (120). UV light is not able to passthrough opaque portion (122) of mask (118) with the result that UVcurable resin that lies below mask opaque portion (22) remains uncuredand flowable for subsequent lamination steps. The mask layer may be anymaterial that is opaque to UV light of the wavelength necessary to curethe UV curable resin. The transparent portions of mask (118) can beprepared by cutting out our removing portions of the opaque maskmaterial to form windows through which UV light can pass. Other methodsknow in the art for creating a mask having UV opaque portions and UVtransparent portions may be used to make mask (118).

The partially UV cured prepreg resulting from the UV light exposure stepis shown in FIG. 5. The partially UV cured prepreg includes UV curableresin layer (102) that has at least one UV polymerized (cured) portion(125) that underlies the UV light transparent portion (120) of masklayer (118) and at least one uncured UV resin portion(s) (127) thatunderlies the opaque portion(s) (122) of mask layer (118). The resin ofthe at least one UV cured portion (125) is essentially fully cured orc-staged and will not flow or be tacky in subsequent lamination steps.

FIG. 6 is a side view of the partially UV cured prepreg of FIG. 5 duringpin lamination of the prepreg to a semi-flexible core (150) including afirst copper layer (154) and a second copper layer (156). In the pinlamination step, optional pins (152) are used to keep prepreg (100) inregistration with at least one semi-flexible core (150). In FIG. 6, core(150) is fully cured. During the pin lamination process, the thermallyhardening epoxy and not the UV exposed resin works as an adhesive in thestack (black arrows) and adheres the layers together only in the areasof the at least one uncured UV resin portions (127) to form a laminatedlayup. The at least one UV cured portion (125) of prepreg (100) is curedor hardened prior to lamination and, as a result, there is no adhesionbetween prepreg (100) and layer (150) in this region because thethermally curable resin cannot flow in this region during lamination. Inthis way, the uncured portion of the prepreg can be removed from thelaminate without first having to create a space (13) in the centralportion of the layup as required in the prior art FIG. 1.

FIGS. 7 and 8 are side views of the laminated layup of FIG. 6 wherein arest material portion (160) of the core material is removed by milling.In FIG. 7, the rest material portion (160) is formed by milling theperimeter (162) of the rest material portion (160) down to the at leastone cured UV resin layer portion (125). The milling creates a perimeteraround rest material portion (160) that lies above or substantiallyabove the at least one UV cured resin layer portion (125). Since restmaterial portion (160) is not adhered to the underlying at least onecured UV resin portion (125), separation of rest material portion (160)from the laminated layup is simplified and can be accomplished withoutfurther milling to form a semi-flexible area (180) of the laminate asshown in FIG. 8.

FIG. 9 is a side view of a multilayer layup including two semi-flexiblecores (170) flanked on both surfaces by prepregs (100, 100′) wherein thethermally curable resin layer (104) of prepreg (100) abuts the surfaceof semi-flexible core (170) and wherein the a UV curable resin layer(102) is outwardly facing. The layup further includes a second corelayer (162) and third core layer (164) each associated with one of theUV curable resin layer surfaces of prepregs (100, 100′) and separatedfrom the UV curable resin layer (102) by an optional layer of copper(164, 164′). Moreover, each core (162) and (164) include an optionaloutwardly facing copper surface (166, 168). The multilayer layup furtherincludes two opposing cavities (180, 182) formed as described above byremoving rest material from the layup as well as a plated through hole(130). As can be seen from FIG. 9, more than one semi-flexible corelayer can be used during the manufacture of printed circuit boardshaving multiple laminate layers.

Prepregs and Laminates

The thermosetting resins and UV curable resins described above areuseful for preparing prepregs shown in side-view in FIG. 3. In order tobe useful in manufacturing printed circuit boards the resins can beformulated and partially cured—or b-staged—to form prepreg sheets. Asprepregs, the sheets can be laid up with additional material sheets toform a “stack” which can be further cured and or laminated to form afully cured laminate sheet. The resulting laminate sheet can be a finalproduct or it can be used as a layer in further printed circuit boardmanufacturing steps. In one useful processing system, prepregs (100) canbe manufactured in a batch or in a continuous process. For example,sheets of partially cured thermal resins can be prepared and then a UVcurable resin can be located on one surface of the partially curedthermal resin sheet to form prepreg (100). Prepregs (100) are optionallymanufactured using a core material such a thermally curable resinimpregnated woven glass fabric. However, if a core material is used iteither can only be associated with a prepreg layer that is opposite thesurface of the prepreg though which the UV light must pass to reach theUV curable resin or it must be sufficiently transparent to UV light toallow the UV light to pass through the core material and reach theunderlying UV curable resin layer. The core material—e.g. woven glassfabric—can be supplied in a rolled up sheet of woven glass web (fabric)which is unwound into a series of drive rolls. The web then passes intoa coating area where the web is passed through a tank which contains thethermosetting resin system of this invention, solvent and othercomponents where the glass web becomes saturated with the resin. Thesaturated glass web is then passed through a pair of metering rollswhich remove excess resin from the saturated glass web and thereafter,the resin coated web travels the length of a drying tower for a selectedperiod of time until the solvent is evaporated from the web. A secondand subsequent coating of resin can be applied to the web by repeatingthese steps until the preparation of the prepreg is complete whereuponthe prepreg is wound onto roll. The woven glass web can be replaced witha woven fabric material, paper, plastic sheets, felt, and/or particulatematerials such as glass fiber particles or particulate materials.

In another process for manufacturing prepregs, thermosetting resins arepremixed in a mixing vessel under ambient temperature and pressure. Theviscosity of the pre-mix can be adjusted by adding or removing solventfrom the resin. The thermosetting resin (varnish) mix can be used tomanufacture unreinforced prepreg sheets and it can also be applied in athin layer to a Cu foil substrate (RCC—resin coated Cu) using slot-dieor other related coating techniques. Thus, it is possible that prepregsused in this invention can include one partially cured thermally curableresin layer having a copper foil on one surface of the sheet. Ifnecessary some or all of the copper foil sheet can be removed to exposethe underlying UV curable resin to a UV light source. Indeed, the copperlayer can be used as the mask layer and portions of the copper layer canbe removed to form UV light transparent portions of the prepreg.

The term “UV curable resin” is used herein to refer to a type of resin—aresin that becomes cured upon exposure to UV light. The term is notintended to indicate the degree of cure of the resin-cured vs. uncured.

The foregoing description of the specific embodiments will reveal thegeneral nature of the disclosure so others can, by applying currentknowledge, readily modify and/or adapt for various applications suchspecific embodiments without departing from the generic concept, andtherefore such adaptations and modifications are intended to becomprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology herein is for the purpose of description and not oflimitation.

What is claimed is:
 1. A method of preparing a laminate sheet comprisingthe steps of: (a) applying a mask to a first prepreg including a UVcurable resin layer and a thermally curable resin layer wherein the maskis applied to the UV curable resin layer, the mask including at leastone UV light transparent portion and at least one UV light opaqueportion; (b) directing UV light at the mask for a period of timesufficient to cure the UV curable resin layer underlying the UV lighttransparent portion of the mask to form a UV light treated first prepreghaving at least one cured UV resin portion and at least one uncured UVresin portion; (c) contacting a core layer with the UV light treatedfirst prepreg such that the at least one cured UV resin portion and theat least one uncured UV resin portion contact the core layer to form alayup; and (d) applying heat and/or pressure to the lay-up to form alaminated layup including a semi-flexible core having a rest materialportion that overlies the at least one cured UV resin portion where thesemi-flexible core layer is not substantially laminated to the at leastone cured UV resin portion.
 2. The method of claim 1 wherein a secondprepreg is laminated to the thermally curable resin layer of the firstprepreg.
 3. The method of claim 2 wherein the second prepreg islaminated to the thermally curable resin layer of the first prepregprior to method step (a).
 4. The method of claim 2 wherein the secondprepreg is laminated to the thermally curable resin layer of the firstprepreg following method step (b).
 5. The method of claim 2 wherein alayer of copper is placed between the second prepreg and the core layer.6. The method of claim 1 wherein the core layer includes a core having afirst and a second planar surface and layer of copper on at least one ofthe first and second core planar surfaces.
 7. The method of claim 6wherein the core layer includes a layer of copper on each of the firstand second core planar surfaces.
 8. The method of claim 1 wherein thelaminate rest material portion of the layup is removed to form asemi-flexible area.
 9. The method of claim 1 wherein the laminated layupincludes a plated through hole.
 10. The method of claim 1 wherein thelaminated layup is incorporated into a printed circuit board.
 11. Themethod of claim 1 wherein the UV curable resin layer has at least one UVlight cured resin portion and at least on UV light uncured resinportion.
 12. The method of claim 1 wherein the thermally curable resinlayer is b-staged.
 13. The method of claim 1 wherein the thermallycurable resin layer is c-staged.
 14. The method of claim 1 wherein alayer of copper is placed between the first prepreg thermally curableresin layer and the core layer.